ACCURATE MEASUREMENTS OF ORGANIC-DYE SOLUTIONS BY USE OF PULSED-LASER PHOTOTHERMAL DEFLECTION SPECTROSCOPY

Organic dye molecules having a finite triplet quantum yield are shown to yield anomalous photothermal signals magnitudes when pulsed laser excitation is used, These anomalous signals can be due to optical bleaching of the electronic singlet states with subsequent optical absorption within the triplet spin state manifold. A model based on excitation and excited state relaxation rate expressions is derived and applied to the analysis of excitation laser irradiance-dependent photothermal signal magnitude data. The model is found to fit the experimental results very precisely. It is subsequently used to correct the photothermal signal at high irradiances and to deduce photodynamic parameters from the studied molecule. Regression of the model to the excitation irradiance-dependent data allows determination of the T-2 --> T-1 relaxation rate constant and the triplet state cross absorption section. Model regression is applied to the analyses of erythrosin in water and ethanol solvent and of eosin and 1,1'-diethyl-2,2'-cyanine iodide dyes in ethanol solutions. A method for quantitative determination of analyte concentration is developed.